On Mon, Aug 15, 2016 at 03:50:03PM +0100, Tom Ellis wrote:
On Mon, Aug 15, 2016 at 03:40:46PM +0100, Malcolm Wallace wrote:
It is no surprise that the instance Eq A is not mentioned, because there is a perfectly valid instance for it already, and it contains no error.
I disagree.
There is no valid instance for Eq (A B), because all that the compiler knows is that 'Eq B => Eq (A B)', and there is no 'Eq B'.
There are two separate issues here.
1. The reason the compilation fails is that there is no instance 'Eq (A B)'.
2. The reason there is no instance 'Eq (A B)' is that there is no instance 'Eq B'.
In my opinion it would be most helpful to the user to mention both 1 and 2. In fact it's likely to be very confusing if you don't mention both of them!
By the way, I agree with the spirit of this:
It points directly to the missing instance, namely Eq B. If you add an instance for Eq B, the program will work ... the error message is correctly steering you away from considering the As, and telling you to consider the Bs instead.
but I don't see why the compiler shouldn't be even *more* helpful in its steering and explain how and why it came to need 'Eq B'. Tom